Re: [PATCH] firmware: stratix10-svc: build only on 64-bit ARM

On Sun, Mar 21, 2021 at 7:46 PM Krzysztof Kozlowski
[off-list ref] wrote:

The Stratix10 service layer and RCU drivers are useful only on
Stratix10, so on ARMv8.  Compile testing the RCU driver on 32-bit ARM
fails:

  drivers/firmware/stratix10-rsu.c: In function 'rsu_status_callback':
  include/linux/compiler_types.h:320:38: error: call to '__compiletime_assert_179'
    declared with attribute error: FIELD_GET: type of reg too small for mask
    _compiletime_assert(condition, msg, __compiletime_assert_, __COUNTER__)
  ...
  drivers/firmware/stratix10-rsu.c:96:26: note: in expansion of macro 'FIELD_GET'
    priv->status.version = FIELD_GET(RSU_VERSION_MASK,

Signed-off-by: Krzysztof Kozlowski <redacted>
Reported-by: kernel test robot <redacted>

While I agree that one shouldn't run 32-bit kernels on this, we should also try
to write drivers portably, and in theory any SoC that can run a 64-bit
Arm kernel
should also be able to run a 32-bit kernel if you include the same drivers.

It seems that the problem here is in the smccc definition

struct arm_smccc_res {
        unsigned long a0;
        unsigned long a1;
        unsigned long a2;
        unsigned long a3;
};

so the result of

#define RSU_VERSION_MASK                GENMASK_ULL(63, 32)
          priv->status.version = FIELD_GET(RSU_VERSION_MASK, res->a2);

tries to access bits that are just not returned by the firmware here,
which indicates that it probably won't work in this case.

What I'm not entirely sure about is whether this is a problem in
the Intel firmware implementation requiring the smccc caller to
run in a 64-bit context, or if it's a mistake in the way the driver
extracts the information if the firmware can actually pass it down
correctly.

        Arnd

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